Method of increasing the opacity of cellulose fibers



April 10, 1962 A. M. THO

MSEN

METHOD OF INCREASING THE OPACITY OF CELLULOSE FIBERS Filed May 18, 1959Calluldss Fiber Jafig'ator Pre 55 50% moisture and lmflreynator '2 Maker30AM! Cb FibflfL 7a Pq nr filndubm Jzz'lb Cal-bower INVENTOR 3,02%,181METHOD (1% INCREASENG THE OPACETY F CELLUDDSE FIBERS Alfred M. Thomsen,265 Buckingham Way, Apt. 402, San Francisco, Calif. Filed May 118, i359,Ser. No. 813,646 3 @laims. (Cl. 162-181) The paper industry isconfronted today with a difficulty inherent in the property ofcellulose, to wit: a compromise between whiteness, of more properlybrightness of the finished sheet, and the desire to have such a sheet ofpaper so opaque that any printing on the back of said sheet will be freefrom show-through, in the language of the printer.

The desire for whiteness leads to ever more drastic bleaching methods,the latest being the use of chlorine dioxide. Unfortunately, the perfectremoval of noncellulose matters by such technique also results in thebleached fiber becoming somewhat transparent. This fault is, in turn,corrected by the addition of pigments, or loading material, varying fromcheap clay to expensive 'tanium dioxide. But unless the amount of suchloading material be kept quite small there will be a serious loss ofstrength in the finished sheet, the reason being, of course, that properbonding of the fibers to one another is adversely efiected by thepresence of such foreign material. Every sheet of paper where bothwhiteness and opacity is desired is, therefore, a compromise between allthese sundry factors.

To understand just how I have corrected this difiiculty it is necessaryfirst of all to consider the structure of the cellulose fiber. ll thefiber, per se, as liberated in pulping and without bleaching, isexamined under the microscope it presents the appearance of a hollowtube, the cell wall itself being often perforated by many tiny aperturesbetween the outside surface and the hollow, central canal. On bleachingthe cell wall itself is attacked. It is an open question just how thelignification that still exists is produced, whether it is a meremechanical mixture or a true chemical combination. Exponents of thelatter theory postulate the presence of a compound calledLignocellulose. Be that as it may, in perfectly bleached pulp there hasbeen a definite attack upon said cell wall and it becomes minutelyscarred by the removal of such material. Simultaneously, the un-wantedtransparency is produced, and it is obvious that a large internal areahas come into existence.

it is the aim and object of my process to coat this internal area withthe loading material and as much as possible to fill the internal poresas well with solid pigment of some type. Naturally, this can only bedone if said pigment is precipitated in situ, any mere admixture ofsolids, in the beater or elsewhere, can never reach the interior of thefiber nor can it really be attached to said fibers. Contrariwise, thesheet can only be a haphazard mixture of fiber and pigment, hence, theinevitable loss of strength. It has long been known that pigmentprecipitated in the beater is far better than a mere mixture, thus: itcalcium chloride be added to the heater and subsequently precipitatedwith soda ash then a better sheet results than if the same amount ofdry, precipitated chalk be added to the beater.

it is well known that the retention of said pigment is enhanced by itsmethod of application, but perhaps the lesson that might have beenlearned is not quite so simple. Obviously, owing to the great dilutionin the beater, stock rarely being above 6%, the precipitate of calciumcarbonate is produced essentially between said fibers and not withinthem, yet there is a tendency in that direction which accounts for theimprovement in the finished sheet. There ice is also some adhesion onthe outside of said fiber which is unobtainable if a dry, finishedpigment be added to the beater.

My process consists in carrying out said precipitation in partiallydehydrated pulp, a substance far too stitf to be mobile in any manner. Iplace the limits as obtained from tests between a pressed sheet havingtwo parts of water to one of fiber and a more strongly pressed sheethaving two parts of fiber to one of water, or approximately between 30%and 70% of moisture as specified in the claims. The first is readilyobtained from the presses of the conventional pulp drier, and the lattercan be obtained by further pressing of said sheet in a differ entialscrew-press, or any screw or hydraulic press. Obviously, the typerequiring the least amount of labor is to be preferred.

In operation, I use a 5% to 20% solution of a salt of the base of theselected pigment, fully saturate the fiber and then press to reach thepredetermined moisture content. A strong solution of the precipitatingchemical is then added and enough time is given so that diffusion willproduce complete reaction between the two solutions. Obviously, theamount of pigment thus added depends upon two factors: to wit, amount ofsolution retained in the fiber on pressing, and the percentage of themetallic salt resident in said solution. After complete reaction, thepulp is washed with water so as to obtain a well washed pulp and minimumdilution of the soluble material which must be recovered as an item ofeconomy and that it may not accumulate in the white water from the papermachine.

As bases I employ either calcium, magnesium, or zinc. As theprecipitating chemical I use always a salt of ammonium, either thecarbonate, the hydroxide and/ or the sulphide. The bases are used eitheras chlorides or as sulphates so the ultimate soluble salt removed bywashing the fiber is always the chloride or the sulphate of ammonium. Iwill now give a specific instance in each case involved. The first ofthese examples is represented on the drawing attached hereto, and inconjunction with the text will be found self-explanatory.

(l) The fiber is first suspended in a 10% solution of calcium chloride,pressed to a moisture content of 50% and sprayed with a concentratedsolution of ammonium carbonate in sufficient amount to precipitate allthe calcium as the carbonate. Sufficient time is allowed toelapse toensure complete precipitation and the fiber is then washed. I prefer todo this with downward displacement but that is optional. The washedfiber is ready for the paper machine and will contain approximately 10%of loading material. Continued washing, with some mechanical violence,will but remove about 2% so it is obvious that the calcium carbonate islargely retained within the strucuure of the fiber. The solution washedout of the pulp is essentially ammonium chloride. Slacked lime is addedto this solution in the conventional ammonia still and ammonia isevolved on heating. The still liquor is a solution of calcium chloride,suitable for impregnating another batch of pulp. The evolved ammonia iscommingled with gases containing carbon dioxide and absorbed in water ina convention coke-packed tower yielding a solution of approximately40%50% ammonium carbonate, suitable for precipitating another batch ofcalcium chloride-impregnated fibers. Calcium carbonate is thus madedirectly from slacked lime, while both chlorine and ammonia is recycledinstead of wasted, making my process very economical, as well as veryeffective. The pulp on drying is very white and opaque, with little lossof strength in spite of the heavy loading.

It will be obvious that magnesium could be substituted throughout forcalcium the loading material thus becoming the even Whiter magnesiumcarbonate. Similarly,

fresh ammonium sulphide.

any intermediate mixture of both could be used. Zinc could likewise beused but there would be a little change in handling the ammonia washwater. It would not be possible to use Zinc oxide direct in decomposingthe ammonia solution, but if lime were used, as herein, then a very lowgrade type of zinc oxide, such as roasted ore, could be commingled withthe calcium chloride solution and carbon dioxide passed through. Theresult would be a very pure solution of zinc chloride, to initiate theimpregnation, and if it were then precipitated withammonium carbonatethe loading material would become zinc carbonate.

(2) The pulp is suspended in a solution of magnesium sulphate of 19%strength, pressed to a moisture content of and sprayed with aconcentrated solution of ammonium carbonate in sufficient amount toprecipitate all magnesium as the carbonate. Sufficient time must elapseto ensure complete precipitation and the fiber is then washed. Saidfiber is ready for the paper machine and will contain approximately 9%of magnesium carbonate. The wash water is evaporated to dryness andfused at approximately 350 C. with evolution of one-half of the residentammonia which is converted to carbonate in water solution as before. Thefused residue is ammonium oi-sulphate which serves in place of sulphuricacid to produce the magnesium sulphate required in the impregnationstep. Any source of magnesia may be used as raw material. In this way,very cheap raw material is made available and the excellence of theimpregnated fiber is much enhanced. Obviously, zinc can be substitutedfor magnesium at will, but calcium cannot be used owing to the slightsolubility of calcium sulphate.

(3) In this example the only metal involved is zinc as the object is toimpregnate the fiber with Zinc sulphide, the pigment which stands nextto titanum dioxide in covering power. The fiber is therefore saturatedwith a zinc solution. Either the chloride or the sulphate may be used.After pressing to the desired moisture content precipitation is madewith ammonium sulphide, best made by commingling ammonia, hydrogensulphide, and water in a coke packed tower as described under Example 1.

After washing the pulp will be ready for the paper machine. Unlessparticularly high opacity is wanted a weaker solution or a harderpressing than previously used will suifice, owing to the far greatercovering power of the zinc sulphide. All such matters are optional. If achloride solution is used then the wash water will be a solution ofammonium chloride. It is commingled with lime in a still, the ammoniadistilled off and used to make The still liquor is a solution of calciumchloride. This is now commingled with some cheap type of zinc oxide,such as a crude roasted ore, washed chimney gas or any gas containingcarbon dioxide is passed until the zinc is in solution as chloride andthe resident calcium precipitated as the carbonate. if the sulphate beselected, then the wash water is evaporated to dryness, further heatedto form the bi-sulphate, the evolved ammonia recovered in water andfurther treated with hydrogen sulphide to form new ammonium sulphide.The bi-sulphate is then dissolved in water, used to dissolve fresh zincfrom roasted ore, the solution conventionally purified until it givesonly a White precipitate with ammonium sulphide, and then re-cycled toprecipitate another batch of fiber. It is manifestly impossible tosubstitute anything for zinc and a sulphide in this example.

Under some exceptional circumstances, it might be desirable toprecipitate magnesium or zinc as the hydroxide by substituting ammoniumhydroxide for carbonate or sulphide, said precipitated hydroxide beingsubsequently further treated to improve its opacity. This may be done bycarbon dioxide, by hydrogen sulphide, or by a water solution of atitanium salt of which the most suitable is the fluoride. Having thusfully described my process, I claim:

1, The method of increasing the opacity of cellulose 4. fiber whichcomprises; saturating said fiber with a 5% to 20% solution of a chlorideselected from the group consisting of calcium and magnesium; removingsuch a portion of the saturating solution that the moisture content ofthe resulting fiber shall be between 30% and 70%; commingling with saidmoist fiber a sufiicient amount of a solution of ammonium carbonate toprecipitate all of the resident metal as the corresponding carbonate;washing the resultant fiber substantially free from ammonium chloride,thus obtaining said ammonium chloride in strong solution; comminglingsaid ammonium chloride solution with sufiicient hydroxide of the metalselected to decompose said ammonium chloride and heating to expelammonia; re-cycling the resulting chloride solution, after adjustment t0the initial strength, as a regenerated saturating solution to freshfiber; commingling the ammonia previously obtained with water and with agas containing.

carbon dioxide to produce a solution of ammonium carbonate forre-cycling to a fresh precipitation of metallic carbonate.

2. The method of increasing the opacity of cellulose fibers whichcomprises; saturating said fiber with a water solution of a salt or" ametal selected from the group consisting of calcium, magnesium and zinc,said solution containing from 5% to 20%, by weight, of the selectedsalt; removing such a portion of the solution that the moisture contentof the resulting fiber shall be between 30% and 70%; commingling saidmoist fiber with a sufiicient amount of a solution of an ammonia saltsuitable for precipitating the resident metallic salt in a substantiallyinsoluble, white, form, within and upon the aforesaid fiber; washing theresultant mixture of fiber and precipitate substantially free fromsoluble ammonia salt, thus obtaining same in a strong solution;separating said ammonia salt into its acid and basic components bycombining the acid portion with a fresh supply of the selected metal,and liberating the ammonia for reuse in forming a regenerated ammoniacalprecipitating liquor; finally re-cycling both combinations to saturateand precipitate a white substance upon fresh fiber.

3. The method of increasing the opacity of cellulose fiber whichcomprises; saturating said fiber with a solution of zinc sulphatecontaining from 5% to 20 of said salt; removing such a portion of thesaturating solution that the moisture content of the resulting fibershall be between 30% and 70% commingling with said moist fiber asufficient amount of a solution of ammonium sulphide to precipitate allresident zinc as the sulphide; washing the resultant fiber substantiallyfree from ammonia salt; evaporating said solution and heating the driedresidue to decompose same into ammonium bi-sulphate and free ammonia;converting the latter into ammonium sulphide for a fresh precipitationand neutralizing the ammonium bi-sulphate in water solution with zinc torc-form a fresh saturating solution.

4. The method of increasing the opacity of cellulose fibers whichcomprises; saturating said fiber with a 5% to 20% solution of a sulphateselected from the group consisting of magnesium and zinc; removing sucha portion of the saturating solution that the moisture content of theresulting fiber shall be between 30% and 70%; commingling said moistfiber with a sufiicient amount of a solution of ammonium carbonate toprecipitate all of the resident metal as the corresponding carbonate;washing the resultant fiber substantially free from ammonium sulphate;evaporating the resultant solution to dryness and fusing the containedsolids with evolution of ammonia and formation of ammonium bi-sulphatein fused form; converting the evolved ammonia into ammonium carbonateand dissolving zinc-bearing material in a solution of the bi-suphateformed in said fusion step; purifying said zinc solution and re-cyclingit to saturate fresh fiber; and re-cycling the solution of ammoniumcarbonate to the precipitation step of the process.

5. The method of increasing the opacity of cellulose J fiber set forthin claim 4, with the added step that the washed mixture of fiber andmetallic carbonate obtained therein be further treated by commingling itwith a solution of titanium fluoride before forming into a sheet.

6. The method of increasing the opacity of cellulose fiber set forth inclaim 2, with the added step that the washed mixture of fiber andprecipitate obtained therein be made still more opaque by cornminglingsaid mixture with a solution of titanium fluoride before it be formedinto a sheet.

7. The method of increasing the opacity of cellulose fibers whichcomprises; saturating said fiber with a 5% to 20% solution of a sulphateselected from the group consisting of magnesium and zinc; removing sucha portion of the saturating solution that the moisture content of theresulting fiber shall be between 30% and 70%; commingling said moistfiber with a solution of ammonium hydroxide to precipitate all residentmetal as the corresponding hydroxide; Washing the resultant fibersubstantially free from ammonium sulphate; evaporating the resultantsolution and fusing the contained solids with References Cited in thefile of this patent UNITED SThTES PATENTS 2,006,016 Eide et al June 25,1935 2,080,437 Rafton May 18, 1937 2,583,548 Craig Ian. 29, 19522,676,884 Hamburg Apr. 27, 1954

3. THE METHOD OF INCREASING THE OPACITY OF CELLULOSE FIBER WHICHCOMPRISES; SATURATING SAID FIBERS WITH A SOLUTION OF ZINC SULPHATECONTAINING FROM 5% TO 20% OF SAID SALT; REMOVING SUCH A PORTION OF THESATURATING SOLUTION THAT THE MOISTURE CONTENT OF THE RESULTING FIBERSHALL BE BETWEEN 30% AND 70%; COMMINGLING WITH SAID MOIST FIBERSSUFFICIENT AMOUNT OF A SOULATION OF AMMONIUM SULPHIDE TO PRECIPITATE ALLRESIDENT ZINC AS THE SULPHIDE; WASH-